Since liquid crystal display (LCD) was discovered, two types of the LCD have been developed and widely used in information display tools, including cell phones, laptops and desktop computers, televisions, and so on. One type is the transmissive LCD which employs a light source called “backlight” at the back side of the liquid crystal cell. The other type is the reflective LCD which uses ambient light as a light source instead of backlight to display an image. Because of using ambient light, the reflective LCD consumes less power than the transmissive LCD so that it is more suitable for portable electronic devices which require low power consumption. However, under the dark ambient, the reflective LCD cannot show the image well. The transmissive LCD, on the other hand, shows the high quality image under the dark ambient because it has its own built-in light source.
To take the advantages and overcome the disadvantages of both transmissive LCD and reflective LCD, the transflective LCD is proposed. Transflective LCD means it can display an image in transmissive display mode and reflective display mode independently or simultaneously. Therefore, such a transflective LCD is designed to be used under any ambient circumstances.
To realize the transflective LCD, some amount of incident light from ambient should be reflected back to the reviewer, and at the same time, some amount of backlight should transmit the LCD device and reach the reviewer's eye independently or simultaneously. The component controlling the reflection and transmission of light is called a transflector hereafter. There are several approaches to realize the function of transflector.
One of the well-known technologies uses a partially transmitting mirror made of very thin metal film. U.S. Pat. No. 4,093,356 issued to Bigelow on Jun. 6, 1978 disclosed a transflective LCD design using partially transmitting mirror. It provides the easiness of designing the device structure. However, to control the uniformity of the metallic film thickness over a large area is not easy. This is especially true for the glass substrate used in the large screen-size LCD manufacturing. Instead of the partially transmitting mirror, the semitransparent reflector which has both fully transmitting part and fully reflecting part has become popular in these days.
U.S. Pat. No. 4,040,727 issued to Ketchpel on Aug. 9, 1977 discloses a transflector based on discontinuous reflective film. An advantage of this kind of transflector is that it can easily control the area ratio of the transmissive part and the reflective part so that it provides the easiness of optimizing the device performance for indoor-oriented or outdoor-oriented applications. FIG. 1 shows a pixel structure 100 of the today's popular conventional transflective LCD which uses discontinuous reflective mirror. It consists of three primary color sub-pixels: Red 101, Green 102, and Blue 103. Each color sub-pixel has a color filter layer and a reflective mirror. Moreover, each sub-pixel further comprises a transmission region, which is denoted as 111, 112, and 113 for each sub-pixel, respectively. Light from the backlight source can transmit through this transmission region and it is responsible for displaying an image in the transmissive mode.
The transflective LCD based on discontinuous reflective film also has some problems, including different electro-optic properties and unequal color reproduction between transmissive and reflective modes. To solve the different color reproduction problem, Fujimori et al. proposed a method using different thickness of color filters for transmissive and reflective parts as disclosed in Digest of Technical Papers of Society for Information Display 2002 International Symposium, p. 1382. This method is effective to make the equal color reproduction for transmissive and reflective images. However, it increases the complexity of the device fabrication process. As for the different electro-optic properties of the transflective LCDs, there are several approaches to overcome this problem.
U.S. Pat. No. 6,281,952 issued to Okamoto et al on Aug. 28, 2001, discloses a transflective LCD which has different thicknesses of the liquid crystal layer on transmissive and reflective parts. In the reflective part, light passes through the liquid crystal layer twice while light in the transmissive part passes through the liquid crystal layer only once. By adjusting the thicknesses of the liquid crystal layer on transmissive and reflective parts, the same optical phase retardation can be obtained in transmissive and reflective parts for both ambient light and backlight. As a result, the equal electro-optic response for transmissive and reflective images can be obtained. However, to fabricate different cell gaps for transmissive and reflective parts, which is also called double cell-gap approach, is not easy.
The '952 patent also discloses using different liquid crystal alignment structures for transmissive and reflective modes. In this configuration, the cell gaps for both transmissive and reflective parts can be identical to each other. Even though this approach reduces the fabrication difficulty of the double cell gap structure; however, the device fabrication process is still not easy due to the complicated alignment process. Another approach without increasing the fabrication difficulty is using double switch devices, such as thin film transistors (TFTs), to control the reflective and the transmissive parts individually and independently, as disclosed by Liu et al. in Proceeding of International Display Manufacturing Conference 2003, p. 215. This technique is called a double TFTs driving method. However, this approach increases the manufacturing cost because it requires twice as many data driver ICs.
For direct view type LCDs, including the transflective LCD, one important technical issue is how to improve the light efficiency so as to enhance the brightness of the image. One of the approaches is to use four sub-pixels, including a white sub pixel, which was proposed by Lee et al in Digest of Technical Papers of Society for Information Display 2003 International Symposium, p. 1212. Such a device design can lower the power consumption by about 50% to achieve the same brightness level as the traditional LCDs. Another approach is to use the color sequential technology to display the color image. U.S. Pat. No. 4,090,219 issued to Ernstoff et al. on May 16, 1978 describes color sequential LCD technology. The basic concept of the color sequential technology is that it displays the color image by sequentially drawn primary color images instead of by the images of primary color sub-pixels. Therefore, the color sequential technology based transmissive LCD can use a color switching backlight and a single pixel without a color filter layer to display a full color image. It avoids the light absorption by the color filter and in the same time increases the pixel aperture size three times for each primary color compared to the conventional transmissive LCD. As a result, the color sequential LCD increases the brightness of images and enhance the power utilization efficiency. Another advantage of color sequential LCDs is improved color reproduction capacity when the light-emitting diode (LED) backlight is used.
However, to realize the color sequential imaging, timing control of the LCD and the driving of backlight is very important. To understand the driving scheme of the color sequential LCD, we need to understand the basic principle of imaging method of the LCD called a line-at-a-time scanning method.
As shown in FIGS. 2a and 2b, a sub-pixel in the conventional LCD consists of a pixel electronic circuit 210 and a pair of electrode and liquid crystal layer 220. The pixel electronic circuit consists of a TFT 211 and a capacitor 212. One terminal of the TFT, called source or data line 201, is connected to the data driver 240 of the system to get image data. One terminal of the TFT, called gate 202, is connected to the gate driver or scan driver 250. The gate signal switches the TFT between the ON and OFF states. When the TFT is ON, the data signal from the data driver transfers to the drain terminal of the TFT which is connected to the capacitor 212. The transferred data signal charges the capacitor 212 and the voltage of the capacitor drives the liquid crystal layer 220.
As shown in FIG. 2b, pixels in the same column are connected to the same data line and all pixels in the same row are connected to the same gate line. The horizontal and vertical sync signals 230 synchronize the signal process between the data driver 240 and gate driver 250. The scan driver 250 selects one gate line each time from the first row to the last row. After the last row is selected, it restarts from the first row again. When one row is selected, the synchronized video signals from the data driver 240 charge the capacitors of all of the pixels on the selected row. As a result, an image is drawn from top to bottom, row by row. Using the capacitor 212, image data are stored during one period of scanning, which is called one frame time. During one frame time, the image is held until it is refreshed in the next frame time. The prior art imaging method described is referred to as a line-at-a-time scanning method.
The line-at-a-time scanning is shown in the timing diagram in FIG. 3. The y-axis represents the row number of the pixels in the LCD while x-axis represents time. Thick slanted lines represent four successive timing lines 311, 312, 313, and 314 for gate line scanning. The time interval between the timing lines of the gate line scanning signal for the same row is the frame period. During the mth frame period, the image data 320 are held. FIG. 3 shows the image data 320 for the first, ith, and Nth rows, respectively. In the m+1th frame period, image data 320 is refreshed by a next image data.
By applying the line-at-a-time driving method to the color sequential LCD, the backlight device exposes red, green, and blue color light with line by line scanning. Each color light remains on during one sub-frame period or slightly shorter. In the next sub-frame period, another different color backlight is turned on and hold for one sub-frame period. Consequently, after three successive sub-frame periods, the red, green, and blue backlight are each turned on once, with one sub-frame period, as shown in FIG. 4. FIG. 4 shows the red area 410, green area 402, and blue area 403 showing the light exposing time for the rows of pixels, respectively. Timing lines of row scanning 411, 412, and 413 are for red 401, green 402, and blue 403 sub-frames, respectively. This kind of backlight device can be used in some specific single panel imager based projection displays, such as digital light processing (DLP) and liquid crystal on silicon (LCoS) systems. However, in the direct-view type LCDs it is very difficult to realize the abovementioned backlight device.
To solve this difficulty, several modified driving schemes were suggested. One of them is using a blinking backlight as shown in FIG. 5. In the figure, red 401, green 402, and blue 403 light are turned on only in a short period, which is much shorter than the sub-frame period. The scanning time of the gate line for a frame image 520 is shorter than one sub-frame period. When the last gate line is selected, the backlight is turned on until the first row is selected again in the next sub-frame period. Therefore, there exists an interval between the first gate line is selected and the last gate line is selected, in which the backlight is turned off. However, the drawback of this method is it requires fast response liquid crystal mode and high intensity backlight source.
Another method is to use the dark sub-frame between two neighboring color sub-frames as shown in FIG. 6. Due to the use of the dark sub-frame, the total number of sub-frames per frame period increases twice compared to the previously described color sequential imaging methods shown in FIG. 4. As shown in FIG. 6, the red 401, green 402, and blue 403 backlights are turned on during two successive sub-frame periods. However, during these two successive sub-frame periods, there is one image sub-frame and one dark sub-frame. Using the red backlight 401 as an example, when the scan driver selects from the first row to the last row, an image sub-frame is inserted following the timing line 613 of row scanning 411. When the scan driver selects the first row again, which is the beginning of the next sub-frame period, a dark sub-frame is inserted following the timing line of the next scanning 611. Using the dark sub-frame, the time intervals of light exposure on all pixels is the same. An advantage of this method is that it is easy to realize the backlight device in direct-view display devices. However, this method also requires faster liquid crystal mode and it suffers half of light energy lose.
U.S. Pat. No. 4,870,396 issued to Shields et al. on Sep. 26, 1989 discloses a liquid crystal display driven by dual switching devices in one sub-pixel. FIG. 7a shows the basic concept of LCD driving based on dual TFTs 710 in one sub-pixel. Each of these two TFTs has its own function: one functions as a memory part to store the image data and the other as an imaging part to control the director orientation of the liquid crystal layer 720 by using the data stored in the memory capacitor 708. In the figure, the data line 703 of the first TFT 701 is connected to the data driver 240, as shown in FIG. 7B. The gate line 704 of the first TFT 701 is connected to the scan driver 250. The drain 706 of the first TFT 701 is connected to the source of the second TFT 702 which is also connected to the first storage capacitor 707.
When the scan driver scans from the first row to the last row, image data are transferred to the first storage capacitor 707 through the first TFT 701. The stored data in the first capacitor 707 do not transferred to the liquid crystal layer 720 immediately because the second TFT 702 is not activated yet during the scanning time. Therefore, the combination of the first capacitor 707 and the first TFT 701 functions as a memory buffer. After scanning all rows, that is, after finishing writing one frame image data into the frame buffer, all second TFTs 702 are activated simultaneously by triggering the gate lines 705. Consequently, the stored image data in the first capacitor 707 are transferred to the second capacitor 708 to control the liquid crystal layer 720. FIG. 7b shows the electrodes connection between sub-pixels and drivers. Because each sub-pixel has two gate input lines G and VS, there are two lines in each sub-pixel which are connected to the scan driver.
The timing chart of the color sequential LCD driving based on the dual TFT method is shown in FIG. 8. During one sequence of the scanning the rows of pixels along the timing lines 411, 412, and 413, the image data of red, green, and blue sub-frame are transferred to the frame buffer memory. After the scanning process, data in the frame buffer are transferred to the second capacitor in the sub-pixels at time points of 801, 802, and 803, as shown in FIG. 8. Color of the backlight is changed synchronously with the time of triggering the second TFTs. During one sub-frame period, the next sub-frame's image data are transferred to the frame buffer memory. The advantage of this method is it doesn't need dark sub-frames. Therefore, it doesn't lose energy of light.